public void AddWorkPlanElement()
        {
            IsWorkPlanElementEditVisible = true;
            EditedWorkPlanElement        = new WorkPlanElement();
            PrepareLists();

            EditedWorkPlanElement.Room      = RoomList.FirstOrDefault();
            EditedWorkPlanElement.Class     = ClassList.FirstOrDefault();
            EditedWorkPlanElement.StartTime = TimeList.First();
            EditedWorkPlanElement.EndTime   = TimeList.ElementAt(1);
            EditedWorkPlanElement.Worker    = Workers.FirstOrDefault();
            _selectedOperation = CrudOperation.Create;
        }